The present invention relates to a method and apparatus for adjusting the compression ratio of internal combustion engines, and more specifically to a method and apparatus for adjusting the position of the crankshaft with eccentric crankshaft main bearing supports.
Designs for engines having eccentric crankshaft main bearing supports have been known for some time. In these engines the eccentric main bearings are rotated to adjust the axis of rotation of the crankshaft. Significant forces bear down on the eccentric main bearing supports during operation of the engine, causing the eccentric main bearing supports to twist out of alignment. Poor alignment of the eccentric main bearing supports is a problem for these engines because even small amounts of main bearing misalignment can cause rapid main bearing failure. Another problem with engines having eccentric main bearing supports is that of a low natural frequency of vibration. Operation of these engines at or near the natural frequency of the eccentric main bearing supports can destroy the engine. The low natural frequency of these engines is a problem because the engines cannot be operated at speeds necessary for use of the engine in passenger cars, trucks, and other applications.
Engines having only one cylinder and two main bearings can tolerate much greater twisting of the main bearing supports, because the crankshaft is free to self align within the two bearings. Single cylinder engines, however, are not employed in the major automobile markets. An objective of the present invention is to provide an eccentric main bearing support for engines having more than one cylinder that provides a long main bearing life, a high natural frequency, and a low manufacturing cost. Another objective of the present invention is to provide an eccentric main bearing support that does not significantly alter overall engine size and mass. Further objectives of the present invention are to provide a compact eccentric main bearing support that permits balancing of primary cranktrain forces and use of a conventional connecting rod having a length no more than two and one quarter times the stroke of the engine.
European patent EP 345-366-A issued to Buffoli Dec. 13, 1989 shows a variable compression ratio engine having a lower main bearing support 30 and an upper main bearing support 41 fastened together with screws 49. The force applied to the main bearing supports causing them to twist is proportional to the cross sectional area of the power cylinder bore and the power cylinder pressure. Main bearing support 30 includes five lower hemispherical disc segments joined by lower webbing. FIG. 1 of EP 345-366-A shows the webbing to have a small cross sectional area relative to the cross sectional area of the power cylinder bore. FIG. 1 also shows that the cross sectional area of the lower webbing is about 3.8% of the projected area of the eccentric member assembly, where the area of the eccentric member is projected on a plane perpendicular to the axis of rotation of the crankshaft. The lower webbing also has a short length, and spans a small arcuate length about the pivot axis of the main bearing support, about 63 degrees. The webbing with its small area and short length fails to provide rigid support of the main bearings. Furthermore, the part has a low natural frequency due to its lack of rigidity. The length and area of the webbing can only be extended downward a small amount without causing mechanical interference with the connecting rod.
Similarly, main bearing support 41 includes five upper hemispherical disc segments joined by upper webbing. FIG. 1 also shows the upper webbing to have a small cross sectional area relative to the size of the cross sectional area of the power cylinder bore. The upper webbing has a short length, and spans a small arcuate length about the pivot axis of the main bearing support. The length and area of the upper webbing cannot be significantly increased upward without causing mechanical interference with the connecting rod. The small cross sectional area of the upper and lower webbing and the small arcuate length of the upper and lower webbing is incapable of maintaining precise alignment of the main bearings, and consequently the main bearings of the engine shown in EP 345-366-A would fail. Furthermore, the main bearing supports have a natural frequency too low for the engine to be commercially viable. The natural frequency is exceptionally low because the webbing shown does not provide a rigid structure and the eccentric discs are massive relative to the size of the webbing. Additionally, because the upper and lower bearing main supports are tightly fastened together with screws, the mass of the upper bearing support is likely to even further lower the natural frequency of the lower main bearing support, and the mass of the lower bearing support is likely to even further lower the natural frequency of the upper bearing support. The outer diameter of the main bearing supports could be increased and the webbing made thicker to increase rigidity, however, the increased mass of the disc segments would adversely effect the natural frequency of the main bearing segments.
Accordingly, and objective of the present invention is to provide, in multi-cylinder engines having eccentricly supported crankshaft main bearings, rigid support and rigid alignment of the crankshaft main bearings at all times to provide a long main bearing life. A further objective of the present invention is to provide a high natural frequency for the eccentric supports to permit operation of the engine over the range of speeds required for commercial use of the engine.
In the present invention, a crankshaft cradle, made up of a large primary eccentric member and small main bearing caps, is employed to rigidly hold the crankshaft main bearings in alignment. The parting line between the primary eccentric member and the main bearing caps is oriented approximately vertically, or approximately parallel with the power cylinder line of action. Additionally, the bearing cap fasteners are located horizontally above (closer to the piston) and below the crankshaft, and the bearing cap bridge thickness minimized in order to locate the crankshaft main bearings in close proximity to the crankshaft cradle outer diameter. According to the present invention, the primary eccentric member is made up of eccentric disc segments rigidly joined by webbing, the arcuate span of the webbing about the eccentric disc segments being greater than 120 degrees, and preferably greater than 150 degrees. The large arcuate span of the webbing is made possible by the large size of the primary eccentric member relative to the main bearing caps, by the vertical orientation of the parting line, and by placement of the crankshaft main bearings in close proximity to the crankshaft cradle outer diameter. According to the preferred embodiment of the present invention, the cross sectional area of the webbing within the 120 degree arcuate span is greater than 35 percent of the cross sectional area of the cradle within the same 120 degree arcuate span. Concurrently the diameter of the primary eccentric member is preferably less than 2.5 times the diameter of the power cylinder and less than 4 times the working diameter of the crankshaft main bearing to provide a high natural frequency. Preferably, at mid span between the eccentric discs the cross sectional area of the webbing is greater than 40 percent of the cross sectional area of the power cylinder. The large contiguous area of the webbing provides a high rigidity and a high stiffness for the primary eccentric member, and precise alignment of the main bearings at all times, which in turn provides a long bearing life, and the small diameter of the eccentric discs provides a light weight and a high natural frequency, permitting operation of the engine over the full speed range required for commercial use of the engine.
The webbing is deeply scalloped towards the eccentric discs to provide further support, to further minimize twisting of the primary eccentric member under firing engine loads and to further increase the natural frequency of the crankshaft cradle. Preferably at one forth span between the eccentric disc segments the cross sectional area of the webbing is at least 20 percent greater than the cross sectional area of the webbing at mid span between the eccentric discs. Preferably the primary eccentric member is a single cast piece, and the webbing is contiguous and has no large holes. Additionally, in the preferred embodiment of the present invention the overall mass of the bearing caps is less than 25 percent of the mass of the primary eccentric member, and consequently the bearing caps cause only a small reduction in natural frequency. According to the preferred embodiment of the present invention, the crankshaft cradle has a natural frequency greater than 100 Hz.